This invention relates to direct current (DC) power supplies and more particularly to overcurrent-protected DC power supplies.
The requirements established by underwriting groups to protect a DC power supply from overcurrent conditions greater than one ampere (including reduced load resistances and short circuits) with quick action are conventionally solved by fusing or by energy-limited transformer design.
The effectiveness of the conventional methods mentioned above are limited. Energy limiting transformer for a power supply and transformer having a normal system demand after rectification of 360 milliamperes at 120 volts DC (150 volts DC open circuit) has drawbacks in the size and cost of the transformer as a means of complying with the one ampere overcurrent protection limit specified by Table 725-31 (b) of the 1975 National Electrical Code.
Fuses are faster-acting than circuit breakers in the vicinity of the latter's trip rating, but result in numerous nuisance outages which require fuse replacement to correct.